how much can i blow with 300g of gun powder?

[Warren Okuma]

Gunpowder is a low explosive, without the concussive effects of high explosives.

In my own Northern Crown rules, a 5-lb. keg (over 2000 grams) of gunpowder does 5d6 damage on a direct hit, 3d6 within 5 feet, and 1 d6 within 10 feet. A grenade (2 lbs, or less than 1000 grams) does 3d6 on a direct hit, 1d6 within 5 feet. I would rule that 300 grams would do no more than 1d6 on a direct hit.

Brown Bess (A musket) needed 85 grains to propel a one ounce musket ball...

That's 4630 grains for 300 grams.

So, you can fire 54 times with a brown bess... Under the right conditions it should do more than 1D6 damage. But if the conditions are wrong 1D6 seems about right.

 

[Celebrim]

With the proper construction you can make a really nice pipe bomb with that. But my suspicion is that most often, without some experience, you'd make something that just fizzles.

In any event, even a nice pipe bomb isn't going to make an explosion which is impressive in D&D terms (due to the whole hit points issue) and the amount of gunpowder you are talking about really isn't all that much.

I'd say 1d6 damage with a 5' 'range increment' and say 3 range increments (close, medium, long). Reflex save (DC 11) for half damage, +2 bonus to save per range increment beyond close. Remember to take into account cover - including someone being behind someone else.